During the period of the first generation colt’s especially (1873-1940) colt was more than happy to accommodate special orders, though personally I fail to see what a 6-6.5 inch barrel would do that the other standard lengths would not, but that could also be said today of many custom guns.
It is certainly possible that one of the Italian imports made a run of a special barrel length.
Another possibility is someone got a squib and damaged a 71/2 “ barrel near the end and cut the barrel back.